Purple colors
often don't appear until the last 2 weeks of flower, sometimes in the very last week of flower in very ripe bud.
Dropping night temps now is a very bad idea, it will possible create extra stretch and will effect yield.
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6 days later
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Genetics mostly determines color as does environment to a much lesser extent, and you should never try force it because doing so will affect quality and yield, as the purple color is a sign of green chlorophyll breaking down and you don't want that happening too soon as that chlorophyll powers your plants carbohydrates and in-turn yield .
Just Grow them the best you can keeping night temps warm, and if they are not purple enough for you, change the strain, many purple strains dont go purple indoors and thats totally ok..